1. Field of the Invention
The present invention relates to an optical fiber comprising at least one polymeric coating crosslinked in the presence of both radical and cationic photoinitiators.
More particularly, the present invention relates to an optical fiber comprising a glass portion and at least one coating of crosslinked polymer material surrounding said glass portion, said coating being obtained by crosslinking a composition comprising at least one radically crosslinkable oligomer containing at least two reactive functional groups; at least one cationically crosslinkable compound containing at least one reactive functional group; at least one free radical photoinitiator; at least one cationic photoinitiator.
Moreover, the present invention relates to a crosslinkable composition that may be applied as said coating.
2. Description of the Related Art
Optical fibers commonly consist of a glass portion (typically with a diameter of about 125 μm), inside which the transmitted optical signal is confined, and of a coating, typically polymeric, surrounding the glass portion for essentially protective purposes. This protective coating typically comprises a first layer of coating positioned directly on the glass surface, known as the “primary coating” or “primary” for short, typically having a thickness of between about 25 μm and about 35 μm. In turn, this primary coating is generally covered with a second layer of coating, known as the “secondary coating” or “secondary” for short, typically having a thickness of between about 10 μm and about 35 μm.
These polymer coatings may be obtained from crosslinkable compositions comprising oligomers and monomers that are generally crosslinked by means of UV irradiation in the presence of suitable photoinitiators. The two coatings described above differ, inter alia, in terms of the modulus of elasticity of the crosslinked material. As a matter of fact, whereas the material which forms the primary coating is a relatively soft material, with a relatively low modulus of elasticity at room temperature, the material which forms the secondary coating is relatively harder, having higher modulus of elasticity at room temperature. The combination of said two layers of coating ensures adequate mechanical protection for the optical fiber.
The optical fiber thus composed has a total diameter of about 250 μm. However, for particular applications, this total diameter may also be smaller; in this case, a coating of reduced thickness is generally applied.
In addition, where one and the same cable internally contains several optical fibers, the operator must be able to identify the different fibers with certainty, hence it is convenient to colour the various fibers with different identifying colours. Typically, an optical fiber is colour-identified by surrounding the secondary coating with a third coloured polymer layer, commonly known as “ink”, having a thickness typically of between about 2 μm and about 10 μm, or alternatively by introducing a coloured pigment directly into the formulation of the secondary coating.
Optionally, the coloured fibers can be then combined into groups of several fibers, typically in the form of fiber optic ribbons, to facilitate their insertion into and recognition within a cable.
Typically from 4 to 24 optical fibers are arranged in parallel to form a fiber optic ribbon and are then covered with a single covering known as the “common coating”, of composition and physical properties similar to those of the secondary coating.
Different crosslinkable compositions useful as coatings for optical fibers have been already disclosed.
For example, patent U.S. Pat. No. 5,262,450 relates to a radiation curable composition comprising about 50-90 parts by weight, preferably about 60-80 parts by weight, of a multifunctional acrylate monomer, suitably with 3-6 acrylates groups, preferably pentaerythritol triacrylate or trimethylolpropane triacrylate; about 10-50 parts by weight, preferably about 20-40 parts by weight, of a divinyl ether monomer having both a hydrophobic backbone and an abrasion-resistant moiety, which is preferably the divinylether of cyclohexane dimethanol, and a radiation curing initiator, suitably a cationic initiator, and preferably, in combination with a free radical initiator. The abovementioned composition is said to provides coatings which are abrasion and water-resistant which may be applied to glass, including fiberglass and glass fiber optics.
International Patent Application WO 97/18493 relates to a removable matrix for retaining one or more optical fibers which have an outermost layer made of a first polymer having a first initiator system, e.g. a free-radical initiated, ultraviolet curable acrylate polymer, the matrix comprising a second polymer having a second initiator system substantially different from the first initiator system, e.g. a cationically initiated, ultraviolet curable vinyl ether polymer. In a further embodiment, the matrix also comprises a partial adhesion promoting amount of a free-radical initiated, radiation curable polymer. The abovementioned matrix is said to permit an easy removal of the fibers from the ribbon.
International Patent Application WO 96/02596 relates to a vinyl ether-containing composition polymerizable by actinic, γ-ray, or electron beam radiation comprising: (a) an oligomer or a mixture of oligomers which are monofunctional, multifunctional, or a mixture of both monofunctional and multifunctional oligomers, and which have a reactive functionality selected from the group consisting of epoxy, acrylate, vinyl ether, maleate, or mixtures thereof; (b) monomers which are monofunctional, multifunctional, or a mixture of both monofunctional and multifunctional monomers, and which have a reactive functionality selected from the group consisting of epoxy, acrylate, vinyl ether, maleate, or mixtures thereof, wherein at least one of (a) or (b) must contain a vinyl ether functionality; (c) a photoinitiator, optionally including a sensitizer, selected from the group consisting of a cationic photoinitiator and a radical photoinitiator; and (d) a thermal oxidation stabilizer. A dual photoinitiator system, i.e having both radical and cationic photoinitiator, may be used. The above-mentioned composition is said to be useful both as a primary coating and as a secondary coating for an optical fiber.
Japanese Patent Application 2000-336127 relates to a photo-curable resin composition comprising: (a) an oligomer which has ethylenically unsaturated groups; (b) a cationically polymerizable diluent monomer; (c) a photo-cationic polymerization initiator; and (d) a photo-radical polymerization initiator. The abovementioned composition is said to have excellent curing properties with a low exposure to ultraviolet radiation, even in the presence of oxygen and, when used as a coating for an optical fiber, it is possible to achieve high productivity. Furthermore, the above mentioned composition is said to have excellent heat resistance and moisture resistance and, consequently, it is said to be particularly useful as a coating material for optical fibers.